Patent Literature 1 discloses a speed reduction apparatus including a first crown gear, a second crown gear, and a third crown gear. The third crown gear is placed with an inclination between the first and second crown gears in such a manner that the third crown gear engages with the first crown gear and the third crown gear engages with the second crown gear. The third crown gear is rotatably supported by an input shaft including a bent portion. The input shaft causes the third crown gear to undergo wave motion in such a manner that the location of contact between the first and third crown gears and the location of contact between the third and second crown gears move in a circumferential direction.
The first crown gear is fixed to a housing. The second crown gear is coupled to an output shaft. When the input shaft is rotated, the wave motion of the third crown gear causes the third crown gear to make rotations equal to the difference in the number of teeth between the first and third crown gears, relatively to the first crown gear. Moreover, the wave motion of the third crown gear causes the second crown gear to make rotations equal to the difference in the number of teeth between the second and third crown gears, relatively to the third crown gear. The speed of rotation of the second crown gear is the combined total of the speed of rotation of the third crown gear relative to the first crown gear and the speed of rotation of the second crown gear relative to the third crown gear. When two pairs of gears (the first and third crown gears, and the third and second crown gears) are rotated in directions where they cancel each other out, a large speed reduction ratio is obtained. When the two pairs of gears are rotated in directions where they encourage each other, a small speed reduction ratio is obtained.